This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The goal of this proposed research is to test the effectiveness of ACTIVE MOMS, a cognitive behavioral physical activity (PA) program, delivered as two interventions (semi-intensive structured exercise and minimum-contact PA), for increasing PA behaviors of women with gestational diabetes mellitus (GDM), and to determine the impact of this program on maternal/fetal outcomes. Because GDM increases maternal/fetal morbidity and it is associated with the onset of type 2 diabetes later in life for these women and their children, it is critically important that interventions be studied to treat and prevent GDM. To date, there are no studies that have intervened with GDM women with a primary objective of increasing their PA behaviors, and limited research examining the effects of maternal PA on pregnancy/delivery complications and infant outcomes. Women will be identified as having GDM by 18 weeks gestation as a result of early screening procedures. Participants will be randomized to receive either the structured exercise, lifestyle PA, or standard care control condition for the remainder of the pregnancy. All participants will undergo multiple assessments over their pregnancy on a series of biopsychosocial variables. Medical records will be obtained to assess pregnancy biomarkers, pregnancy/delivery complications, and infant birth outcomes. The primary outcome is women's PA behaviors (e.g., mean counts/min, energy expenditure, meeting PA guidelines). Secondary outcomes include the effects of maternal PA on psychosocial correlates (e.g., physical activity attitude, self-efficacy, social support;depression, quality of life), pregnancy/delivery complications (e.g., maternal weight status, need for insulin), and infant outcomes (e.g., birth weight.